1. Field of the Invention
The present invention relates to a lens device having a lens which moves to predetermined positions in an optical axis direction.
2. Description Related to the Prior Art
Mobile phones having a built-in digital camera are widely used. In the digital camera, there is a type which switches between ordinary photographing and close-up photographing. In a mobile phone 10 shown in FIG. 8, a lid 11 having a liquid crystal display and a main body 12 having a keyboard are folded around a hinge 16 such that the liquid crystal display faces the keyboard. In the lid 11, a camera window 13 is provided on the rear of the liquid crystal display, and a lens device is arranged between the camera window and the liquid crystal display. A slide lever 14 is provided on a side surface of the lid 11, adjacent to the camera window 13. By operating the slide lever 14, a taking lens 15 of the lens device is drawn out toward an objective side to shift to a close-up photographing position from an ordinary photographing position along an optical axis 15a. 
As shown in FIG. 9 and FIG. 10, a conventional lens device 17 is constituted of a lens holding frame 18, a cam ring 19, a fixed frame 20, an infrared ray cutting filter 21 and a light shielding frame 22, sequentially in this order from the objective side. A Charge Coupled Device (CCD) is fixed at the rear of the light shielding frame 22. The lens holding frame 18 holds the taking lens 15, which is a fixed focal length lens.
On the rear of the cam ring 19, a cam surface 23 is formed along its circumferential direction. The cam surface 23 contacts a cam pin 24 provided on the fixed frame 20, from the direction of the optical axis 15a. The cam ring 19 is rotatably arranged between a fixed barrel 20a and the lens holding frame 18 which are arranged along the direction of the optical axis 15a. In addition, the cam surface 23 is biased to contact the cam pin 24 by a tension springs 25 which pulls the lens holding frame 18 to the fixed frame 20. A projection 26 is provided on outer periphery of the cam ring 19. The projection 26 is connected to the slide lever 14, accordingly the cam ring 19 is rotated at predetermined angle to move the lens holding plate 18 along the optical axis 15a by operation of the slide lever 14. Projection rods 27 which protrude toward the objective side are provided on each of four corners of outer periphery of the fixed frame 20. These projection rods 27 stop the rotation of the lens holding frame 18 by contacting with a hook 28 of the lens holding plate 18. And spaces between the projection rods 27 house the tension springs 25.
To house the lens device 17 in small mobile devices like the mobile phone, there is a need to use the fixed focal length lens for simplifying a focus adjustment mechanism, so that the mobile phone becomes more compact and reduces its cost. Especially, there is a need to reduce the thickness of the lens device in the direction of the optical axis 15a, because the optical axis of the taking lens is perpendicular to display plane or operation plane of the mobile phone. However, the lens device 17 stated above, has large length in direction of the optical axis 15a, because the fixed frame 20, the cam ring 19, and the lens holding frame 18 are overlapped along the optical axis 15a. And the diameter of the lens device 17 becomes larger by providing the space for the tension spring 25. Further, the lens device has low assembling efficiently because there is a need to hang the spring 25 in assembling. In addition, the spring 25 is sometimes unhooked by a hard impact or the like to cause malfunction.